Aqueous synthetic rubber emulsions having a high level of solids (herein referred to as "high-solids synthetic rubber emulsions") are very important industrially for the production of shaped foam articles and foamed carpet backings. It is known that the latex particles in such latices must be relatively large and their size distributions must be relatively broad in order to obtain a high solids content while maintaining a low viscosity. For example, the preparation of a high-solids aqueous styrene/butadiene emulsion having a solids content of up to 70% by weight directly by conventional emulsion polymerization is not possible since finely dispersed emulsions are obtained in the conventional industrial emulsion polymerization processes. Due to the small particle size, the viscosity of the emulsion increases very sharply at such high solids levels.
Although the preparation of coarse-particled emulsions is known (see DE 21 65 410), such coarse-particled emulsions are not of industrial interest in the case of styrene/butadiene emulsions, because as the particle size increases and hence the particle concentration decreases, the rate of polymerization decreases considerably at the same phase ratio. This causes the process to become uneconomical due to the extremely long reaction times thus required.
Finely dispersed synthetic rubber emulsions are therefore usually prepared with a relatively low solids content and converted by agglomeration into coarse-particled synthetic rubber emulsions having a broad distribution of particle sizes. High-solids synthetic rubber emulsions are then obtained by subsequent removal of water.
Conventional processes for agglomeration are pressure agglomeration and agglomeration by freezing. In these methods, the application of high pressure or partial freezing of the synthetic rubber emulsions causes partial colloidal instability which leads to agglomeration of the latex particles. As a result, the viscosity of the synthetic rubber emulsion is substantially reduced at high solids levels. Both processes are technically very demanding and therefore contribute to high operating costs.
Another process is chemical agglomeration, in which an agglomerating agent, which spontaneously initiates a controlled agglomeration process, is added to the finely dispersed synthetic rubber emulsion. Such a process is described in DE 26 45 082 and U.S. Pat. No. 3 330 795. Oxidized polyethylene oxides have been traditionally used as agglomerating agents. This conventional process provides the agglomeration of finely dispersed synthetic rubber emulsions to give latices having a very broad distribution of the particle sizes and a number average particle diameter ranging from 0.2 to 2 .mu.m. Such synthetic rubber emulsions can be concentrated to solids contents of 65 to 70% by weight at a Brookfield viscosity level of 1,000 to 3,000 mPa.s, which is acceptable for industrial use.
However, it is desired to obtain agglomerating agents which can be used to provide high solids synthetic rubber emulsions with improved costs of production, improved shelf life and which do not exhibit separation of coarse particles.